Split prepayment meter in a BS housing.The Gemini is a single-phase, keypad-based, split prepayment electricity meter in a British Standard housing comprising two parts, namely, the customer interface unit and the meter. The meter is usually installed in a secure, locked enclosure, typically a pavement kiosk or pole-mounted equivalent. The customer interface unit is the customer's only interface with the meter and is a compact unit with a user-friendly keypad and display. It is usually installed in a convenient location in the consumer's home - remote from the meter - and is connected to the meter with a pair of communications wires.
The customer interface unit with its user-friendly keypad and display is usually installed in a convenient location inside the consumer's home - remote from the meter - and is connected to the meter with a pair of communications wires up to a distance of 130 metres.
The meter contains all critical metering, number decryption and load control functionality. It operates independently and is immune to any form of tampering on the customer interface unit.
The meter is usually installed in a secure, locked enclosure, typically a pavement kiosk or pole-mounted equivalent. It is outside the consumer's home to facilitate easy inspection by the utility at any time and to reduce the risk of fraud and tampering.
As an option, the meter can be supplied with its own LCD display and keypad which allows the utility to view important meter parameters without the need for an interrogation tool.
The Gemini meter provides utilities with the utmost flexibility in terms of being able to adapt to a range of different consumer profiles. Three utility-programmable modes of operation are available and it is possible to switch between modes as required, that is:
In Prepayment metering mode it functions as a normal prepayment meter. Credit tokens are purchased and entered into the meter via the customer interface unit keypad. On expiry of credit the load is disconnected and will only be re-connected when a valid credit token, purchased by the consumer, is entered.
In Credit metering mode it functions as a conventional credit meter. Power is continuously supplied to the consumer and total kWh used is continuously measured and recorded. The meter must be read by the utility at regular intervals and the consumer billed accordingly.
This mode allows utilities to distribute a fixed, monthly allocation of energy to consumers. It encourages the rational use of energy without severely inconveniencing the consumer.
Operation is as follows:
Assume that a monthly energy allowance of 150kWh has been allocated to a consumer. The meter allocates this energy in regular, equal portions over the thirty-day period i.e. by incrementing the kWh credit level with a value of 0.00087kWh every 15 seconds.
Assuming that the consumer draws no power at all, the credit level will continue to increase. However, as soon as energy starts to be used, the credit level is proportionately decremented. If the rate at which energy is being used is less than the rate at which it is being incremented, the credit level will slowly continue to increase. If the rate at which energy is being used is greater than the rate at which it is being incremented, the credit level will slowly decrease. It is in the consumer's interest to ensure that electricity is not wasted and that unnecessary appliances are turned off. By conserving energy, it will be possible to use it at a high rate for periods when required.
In the event of the consumer exceeding the allocated allowance (credit level reduced to zero), the load is disconnected. However, the next allocation of credit will be available within a very short period of time (15 seconds) and the supply of electricity restored. Providing the consumer takes immediate steps to disconnect unnecessary appliances, it will be possible to have at least basic services available e.g. lighting. With a 150kWh monthly allocation of energy, it will be possible to maintain a continuous load of 200W whilst still maintaining a positive credit balance.
User interaction with the meter and access to meter information (such as a low credit warning, energy consumption and load contactor status) is available using the customer interface's keypad and large LCD display. The customer interface makes use of clear, language-independent icons.
In addition, various audible tones are sounded under different conditions (e.g. Low Credit Alarm).
As an option, the meter can be supplied with a local keypad and LCD display. This allows the utility staff to inspect and update various parameters of the meter in the remote kiosk without having to gain access to the consumer's house to access the customer interface unit.
The meter includes an LED status indicator. This allows a technician to view the state of the meter without the need for specialised interrogation tools or having to gain access to the consumer's premises. Information such as tamper status, power limiting, commissioned status and credit status are available.
The meter also features a dedicated diagnostic LED for indicating the status of the customer interface unit communications. It can indicate the presence of "Open" or "Short" circuited communication lines. This is a valuable visual aid that assists the technician to validate the installation and determine probable fault types.
More detailed information and programming is achieved via the standard interrogation port at the rear of the meter.
As a standard feature, the Gemini family offers an IEC 62056-21 compliant optical communications port. This allows the utility to access a variety of information stored inside the meter.
The split configuration of Gemini meters significantly reduces the risk of tampering. The meter is installed in a remote, secure location and is mechanically sealed against tampering through the use of a factory-sealed screw plug on the rear panel, and a utility-sealed wire seal on the front of the meter. The use of these mechanical seals ensures that there are visible signs of tampering if unauthorised entry to the system is attempted.
In addition, the units are equipped with a terminal cover tamper sensor that will automatically disconnect the power to the load in the event of tampering.
The meter also has a feature allowing detection of Significant Reverse Energy (SRE). If the line and load wires are swapped during installation, the meter will continue to operate and decrement credit but can be factory programmed to tamper and disconnect the load should SRE be detected.
The Gemini family has the option of being supplied with a built in surge arrestor that is capable of sustaining up to 30kA during transients
The communications interface can withstand voltage surges of 6kV, however it is recommended that one of the communication lines be earthed at the meter for additional protection.